This invention relates to optical attenuators and more particularly to optical attenuators suitable for use with intense light sources such as high-energy lasers.
Optical attenuators are a standard tool used in most optical work. However, when the optical system involves high-energy lasers, controlled attenuation for prolonged periods is difficult to achieve.
Optical attenuation has previously been accomplished in various way, none of which has proven to be adequate when used in conjunction with high-power lasers. These ways involve the use of (1) bulk absorption or reflective attenuation plates, also known as neutral density filter; (2) Glan-Laser prisms; and (3) Brewster wedges.
A reflective attenuation plate consists of a thin, semi-transparent metal film placed on or between two sheets of glass. These do not work at high-power levels because the metal film tends to burn off. A bulk absorption attenuation plate consists of semitransparent glass that attenuates by absorbing a portion of the incident light in transmission. At high incident powers these fail by optical bleaching or fracture.
A Glan-Laser prism consists of two birefringent prisms separated by an air space. When used for attenuation two of these prisms are required. Rotating one prism about its optical axis with respect to the other will reduce the transmission through both. This is equivalent to the transmission decrease observed through crossed polarizers. However, when a high-power laser beam passes through the transparent material of the prisms, localized heating causes spatial distortion in the transmitted beam. This is not acceptable for applications where spatial beam coherence is required. A 1/2 wave waveplate can be used instead of one prism and this can lower the level of distortion. However, Glan-Laser prisms are not suitable for use at very high power levels and produce a linearly polarized laser beam.
Brewster wedges are made of transparent materials such as glass, SiO.sub.2, Al.sub.2 O.sub.3, CaF.sub.2, or Ge. It is known that when the angle of incident of light is at Brewster's angle on one of these materials, the transmitted light is unattenuated. As the angle of incidence is changed from Brewster's angle the transmitted light is reduced by Fresnal reflections, typically 3 to 4% for these materials per surface in the visible and near infrared. Wedges are used to eliminate interference between the Fresnal reflections. If these are to be used in a non-distorting attenuator with a large dynamic range, a large number of wedges must be used, and these must be carefully aligned. This tedious and expensive and produces a transmitted beam that varies in polarization as the angle of rotation varies.